Add Nil constructor and tests

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/builtins.rs

@@ -569,3 +569,13 @@ impl Bool {
         Float::create(bool_value as i64 as f64).into()
     }
 }
+
+impl Nil {
+    pub(crate) fn do_call(_vm: &mut Vm, _state: FunctionState) -> FunctionResult {
+        FunctionResult::ReturnPush(Nil::create())
+    }
+
+    pub(crate) fn init(_vm: &mut Vm, _state: FunctionState) -> FunctionResult {
+        FunctionResult::ReturnPush(Nil::create())
+    }
+}

src/obj.rs

@@ -221,7 +221,15 @@ pub fn init_types() {
 
             // Operators
         },
-        Nil { },
+        Nil {
+            // Conversion methods
+
+            // Constructor
+            __call__ => BuiltinFunction::create("__call__", Nil::do_call, 1),
+            __init__ => BuiltinFunction::create("__init__", Nil::init, 1),
+
+            // Operators
+        },
         BuiltinFunction { },
         UserFunction { },
         Method { },